What kind of flu vaccines are there?
There are two types of vaccines that protect against the flu. The "flu shot" is an inactivated vaccine (containing killed virus) that is given with a needle, usually in the arm. A different kind of vaccine, called the nasal-spray flu vaccine (sometimes referred to as LAIV for Live Attenuated Influenza Vaccine), was approved in 2003. The nasal-spray flu vaccine contains attenuated (weakened) live viruses, and is administered by nasal sprayer. It is approved for use only among healthy people between the ages of 5 and 49 years. The flu shot is approved for use among people over 6 months of age, including healthy people and those with chronic medical conditions.

Each of the two vaccines contains three influenza viruses, representing one of the three groups of viruses circulating among people in a given year. Each of the three vaccine strains in both vaccines -- one A (H3N2) virus, one A (H1N1) virus, and one B virus -- are representative of the influenza vaccine strains recommended for that year. Viruses for both vaccines are grown in eggs.

How do flu vaccines work?
Both flu vaccines (the flu shot and the nasal-spray flu vaccine (LAIV)) work in the same way; they cause antibodies to develop in the body, and these antibodies provide protection against influenza virus infection.

Why should people get vaccinated against the flu ?
Influenza is a serious disease, and people of any age can get it. In an average year, the flu causes 36,000 deaths (mostly among those aged 65 years or older) and 114,000 hospitalizations in the United States. The "lu season" in the United States is usually from November through April each year. During this time, flu viruses are circulating in the population. An annual flu vaccine (either the flu shot or the nasal-spray flu vaccine) is the best way to reduce the chances that you will get the flu.

When should I get a flu vaccination?
Beginning each September, the flu shot should be offered to people at high risk when they are seen by health-care providers for routine care or as a result of hospitalization. See the ACIP Recommendations for Using Inactivated Influenza Vaccine.

The best time to get vaccinated is from October through November. Flu activity in the United States generally peaks between late December and early March.

You can still benefit from getting vaccinated after November, even if flu is present in your community. Vaccine should continue to be offered to unvaccinated people throughout the flu season as long as vaccine is still available. Once you get vaccinated, your body makes protective antibodies in about two weeks.

Does flu vaccine work right away?
No. It takes about two weeks after vaccination for antibodies to develop in the body and provide protection against influenza virus infection. In the meantime, you are still at risk for getting the flu. That’s why it’s better to get vaccinated early in the fall, before the flu season really gets under way.

Can I get the flu even though I got a flu vaccine this year?
Yes. The ability of flu vaccine to protect a person depends on two things: 1) the age and health status of the person getting the vaccine, and 2) the similarity or "match" between the virus strains in the vaccine and those in circulation.

How effective is the flu vaccine?
The flu vaccine is the most effective way to prevent the flu. However, in limited studies, the flu shot and the nasal-spray flu vaccine (LAIV) have different rates of effectiveness.

Why do I need to get vaccinated against the flu every year?
Flu viruses change from year to year, which means two things. First, you can get the flu more than once during your lifetime. The immunity (natural protection that develops against a disease after a person has had that disease) that is built up from having the flu caused by one virus strain doesn’t always provide protection when a new strain is circulating. Second, a vaccine made against flu viruses circulating last year may not protect against the newer viruses. That is why the influenza vaccine is updated to include current viruses every year.

Another reason to get flu vaccine every year is that after you get vaccinated, your immunity to the disease declines over time and may be too low to provide protection after one year.

How are the viruses for flu vaccine selected?
Each year, many laboratories throughout the world, including in the United States, collect flu viruses. Some of these flu viruses are sent to one of four World Health Organization (WHO) reference laboratories, one of which is at the Centers for Disease Control and Prevention (CDC) in Atlanta, for detailed testing. These laboratories also test how well antibodies made to the current vaccine react to the circulating virus and new flu viruses. This information, along with information about flu activity, is summarized and presented to an advisory committee of the U.S. Food and Drug Administration (FDA) and at a WHO meeting. These meetings result in the selection of three viruses (two subtypes of influenza A viruses and one influenza B virus) to go into flu vaccines for the following fall and winter. Usually, one or two of the three virus strains in the vaccine are changed each year.